One of the problems with any WiFi enabled device is the initial configuration. How do you tell your device the settings for your WiFi when you can’t talk to it via WiFi? It’s a “chicken and egg” situation, even though it doesn’t involve any chickens or eggs.

One answer is to make your device set up as a WiFi access point of its own, and then connect to this from a WiFi enabled phone or computer to configure all the settings. There are quite a few libraries of routines out there that will help you do this, but none of them did exactly what I wanted, so I’ve built my own.

You can see the configuration screen it generates in the screenshot above. You connect to it via your browser and then fill in the various options. It has support for five different access points and also lets you configure all the MQTT settings for a sensor as well.

I’ve got it working with my MQTT Mini Air Quality sensor. To configure the WiFi you simply turn the sensor upside down, plug it in and it starts up in “configure” mode. You then connect to the access point and perform the configuration. Then flip the sensor the right way up, restart it and away you go.

I’m very pleased with how well it works. I’ll have the code for the new, super configurable MQTT Mini device on GitHub before the end of the week. Then I’ll port the configuration mechanism over to larger, ESP32 based sensor.

But actually for me it’s a thing. I’m getting my head around MQTT as a way of sending data from our air quality sensors to the systems that want to work on the data. MQTT is a “publish and subscribe” transport. An MQTT node can send messages to a particular address managed by sn MQTT broker process running on a machine somewhere. Other processes can subscribe to that address on the broker and, when the mode sends a packet of data (string of bytes) to the that address the broker will then send that message to everyone who has subscribed to that address.

Make sense? OK, think of it as a mailing list for a pop star, perhaps David Essex. Anyone interested in what David is up to can subscribe to the mailing list, which has a particular name. David’s publicist, who produces the content describing the amazing life that I’m sure David has, (other pop stars are available for this exercise) connects to the mailing list as a publisher, and sends messages which are then sent out to all the subscribers. So MQTT Is basically a fan mail management system.

The address expressing the a particular endpoint is called a topic. Topics are hierarchical. So, an air quality sensor could send a messages to the following topics with the respective data items from that sensor:

Sensor01/tempSensor01/pressureSensor01/humidity

Alternatively, an air quality sensor could send a single message containing all three values, encoded as a lump of JSON (JavaScript Object Notation) to a single endpoint:

Sensor01/data

Which is better? I’m not sure. Object Oriented Rob says that having all the related items in a single cohesive object which can also contain things like timestamp and location information will make it much easier to process. Free Wheeling Rob says that having individual data on separate topics makes it possible to handle data produced at different rates by different sensors, and also makes it easier for something only interested in temperature (a fan perhaps) to just subscribe to that data item. And both Robs need to remember that the broker is just that, a broker. It doesn’t have any bearing on what will happen to the values that are passed through it from a device to a process that is doing something with it.

As usual, when faced with a problem like this, I’ve gone a bit meta. To me the important thing is probably not which particular way the values are configured for a given application, but that it should be easy to change this configuration during the lifetime of a node. To this end I’m working on a little protocol which will let a node advertise the readings that it has available and then be told which readings to be sent where. Once a node has been told where to send its data it will do that until told otherwise. This means that we can change how we use the sensors so that we could use either of the above approaches.

The good news is that this will be wonderful. The bad news is that I’ve got to do it. Oh well. Today I got a Python program talking to MQTT. The next step is to get JSON working on the Arduino so our systems can start exchanging configuration information.

robmiles.com

Rob Miles is technology author and educator who spent many years as a lecturer in Computer Science at the University of Hull. He is also a Microsoft Developer Technologies MVP. He is into technology, teaching and photography. He is the author of the World Famous C# Yellow Book and almost as handsome as he thinks he is.